The removal of asbestos materials from buildings has evolved into a procedure with fairly standard practices and environmental controls. These have been reinforced by recent State and Federal (OSHA) regulations controlling the construction and renovation industry which now mandate practices of isolation, HEPA (high efficiency particular air) air filtration, and establishing a negative pressure enclosure. Prior art, such as the patent to Natale, U.S. Pat. No. 4,604,111, also teaches the use of negative pressure in the removal of asbestos.
A typical asbestos removal site is prepared by sealing all penetrations into the work area and covering floors, walls, and horizontal surfaces with plastic sheeting. An artificial "bubble" is thus created into which there is only one entrance which serves both as the worker's access and decontamination facility. Fans with highly efficiency HEPA filters are situated within the work area to exhaust air from within the enclosure to the surroundings. Make-up air is provided through the worker's access/decontamination unit, and the constant exhausting of large volumes of filtered air from within the work area relative to the much smaller amounts of make-up air admitted through the decontamination unit creates a negative static pressure relative to the surrounding spaces. The dual features of these ventilation units, namely--the production of a negative static pressure within the enclosure and the air filtration capability of the HEPA filters, has caused an unclear perception with regard to the actual purpose and applicability of this widespread engineering technique. This standard feature of asbestos work zones is presently not clearly defined from the perspective of industrial hygiene ventilation and retains features of an industrial hygiene as well as an environmental control. Consideration of the "negative air" concept as industrial hygiene ventilation allows a more clear description of its capabilities and limitations, as well as enabling alternatives from current practice in controlling airborne asbestos.
The basic concept and empirical derivations of industrial hygiene ventilation that are standard today have evolved over the last forty or fifty years. The variety of aerosols and vapors to be controlled in classical industrial hygiene settings have generally been incidental to the formuation of some desired material, and engineering solutions to these airborne hazards have taken two approaches. General dilution ventilation (the first approach) as been most applicable where the generation of a relatively low hazard contaminant evolves from such a widespread area that point control by local exhaust at the source is impractical. An example of this would be the general ventilation necessary to maintain an office environment free of excessive cigarette smoke.
The object of the general dilution type of control method is to bring enough fresh air into an area to reduce airborne concentrations to some acceptable concentration, which is either some guideline value, regulatory standard, or comfort level. Factors such as contaminant characteristics (e.g., toxicity), quantities generated, seasonal variations, and building configuration have led to standardized formulas describing quantities of air necessary for this type of control.
Local exhaust ventilation, the second fundamental technqiue of industrial contaminant control, attempts to confine a contaminant-generating process as much as possible within an enclosure termed a hood. Through the use of exhaust fans and the hood configuration, the contaminant is captured as close as possible to the source. From there it can be channeled via ductwork to some location where the toxic agent can be controlled for disposal or some appropriate treatment. Recognizing that for effective capture, certain hood/ductwork design and fan capacities are necessary to create enough "capture velocity" for a given contaminant, several empirically derived design formulas are currently the industrial hygiene engineer's guidelines in designing the local exhaust system.
These two approaches characterize the industrial hygienist's attempts at contaminant control and the distinction between the two has usually been clearly defined. In the asbestos control industry however, standard negative air ventilation techniques as well as the features of the typical work site are somewhat different from the seen in the typical industrial or manufacturing setting. The mose obvious difference is that a typical asbestos control site is in a non-industrial structure. This not only requires controls of the obvious occupational exposure within the work containment, but also necessitates that the airborne asbestos dust be confined so that space to protect the surrounding environment.
Additionally, because asbestos control (i.e., removal projects often are conducted in occupied buildings, the potential for exposure of other persons outside the asbestos removal work area to asbestos dust (if not effectively contained) initiates concerns for non-occupational exposure. While there are presently no legal standards for such exposure, the liability implications are far-reaching. Therefore, the very nature of an asbestos project mandates control over the occupational exposure within the work area as well as an environmental control to prevent contamination into the adjacent (and often occupied) spaces. An apparent conflict thus arises, since confining airborne asbestos to the work area inherently produces an increased exposure to personnel working in that area. In turn, increased exposure caused by the confinement necessitates more cumbersome personal protective equipment, and results in a reduction in worker productivity.
Because the common operation of negative pressure enclosures is based upon exhaust fans drawing make-up air through the decontamination unit entrance from the adjacent clean spaces, the system superficially performs the function of dilution ventilation. Current industrial hygiene practice dictates that dilution ventilation is acceptable for low hazard solvents in which quantities of fresh air will lower the concentration of a contaminant below a certain acceptable level. The quantities of air necessary can be calculated since the rate of contaminant (vapors) generation is generally predictable for a specific operation and the air can be distributed to localized areas via ductwork. The nature of asbestos removal, however, is such that the rate of airborne asbestos fiber generation is seldom constant due to the variety of asbestos-containing building products encountered. Even if dilution were applicable to particulates rather than vapors, the dilution capacity that may be adequate in maintaining a specific work area airborne asbestos particulate concentration while removing low asbestos percentage acoustical plaster will not maintain the same airborne concentrations when removing a high percentage deck fireproofing. (Asbestos fireproofing for example, may result in short term personnel exposures from the action limit to 100 fibers/cc depending upon work practices.) Architectural configuration of work zones is another consideration which makes dilution capacity difficult to determine, since office partitions, corridors,, etc., influence air flow and fresh air mixing, and vary greatly from one worksite to the next.
The fact that large quantities of air are exhausted and filtered during the negative pressure asbestos removal process has led to a misconception that the primary purpose of the air filtration is to "clean the air" and thereby reduce the worker exposure. While this may occur to a limited degree, the dilution ventilation capability of the typical negative air arrangement is inferior for reduction of worker exposure since the contaminant of concern, asbestos, has a high toxicity, the generation rate is highly variable, and the asbestos exposure is a result of several point sources within a large area. This is especially true where a high asbestos percentage surface coating (e.g., fireproofing) is being removed.
If asbestos removal is the enclosed work area is analogous to working in a large hood (i.e., enclosed process with exhaust fans), then consideration of the negative air enclosure can be made in terms of local exhaust ventilation. Air filtration units are commonly placed at convenient locations within the work area, usually at the perimeter with an exhaust duct leading to a "clean" area outside the work zone. In the alternative, the intake force of the filter may protrude through the containment or isolation barrier of the work zone, while the bulk of the machine which houses the HEPA filter fan and exhaust duct is situated in the adjacent "clean" space. This latter arrangement facilitates cleaning of the unit at the end of work (as opposed to its location within the work areas); regardless of the position of the filter, the velocity of intake air, which defines the ability to capture the generated asbestos dust, is virtually non-existent at any substantial distance from the face of these units. One need only visualize air flow via smoke at various distances to verify that the capture velocity into the unit is negligible. Perimeter placement of the air filtration device as described above can be characterized, in terms of local exhaust ventilation, as a flanged hood. The air velocity into such a hood is described by the formula: ##EQU1## where: Q=volume of air exhaust in cubic feet per minute
A=area of the hood opening (approximately 3.35 square feet for the common 22 inch square intake) PA1 x=distance from the hood in feet PA1 V=velocity of air at distance X in feet per minute
The two areas of highest air velocity in a work area are the decontamination entrance (theoretically the only make-up air inlet) and the intake face of the air filtration devices. The distance between these two locations is characterized by a negligible flow of air with "dead spots" (a common problem in dilution ventilation) and virtually no air exchanges depending on the dimensions and non-uniformity (i.e., alcoves, office partitions, etc.) of the work area.
Considerations up to this point have been with regard to quiescent, inactive conditions. However, when one considers the high activity in the asbestos removal zone (work area) and the fact that simply walking at a normal pace generates air flow of 50-70 feet per minute (fpm), it is quite unreasonable to expect air filtration devices to substantially reduce personnel exposure when their "capture velocity" is 140-180 feet per minute at a one foot distance, and less than 75 feet per minute at two feet from the intake. (This can be compared to the recommended velocity for local exhaust for an enclosed asbestos debagging operation in industry of 200 feet per minute in an enclosed hood.)
The design of negative air systems too often only gives consideration to total air volumes exhausted without recognizing the characteristics of exhaust ventilation. The proximity of the exhaust units to the worker's removal activity within the work area is more a determining factor than total air flow if a local exhaust capability is desired. Unfortunately, workmen are not inclined to position air filtration devices close enough (i.e., within a foot) to the actual removal activity for effective collection of airborne asbestos dust. While the filtration devices can be equipped with an intake manifold and extended flex duct (12 inch diameter), the flow of air into a round open duct is described as ##EQU2##
This provides at best a capture velocity of 140-180 feet per minute at a one foot distance from the dust opening, and less than 50 feet per minute at a distance of two feet. Thus, normal work activity negates any local exhaust ventilation capability for most asbestos work areas, since no substantial velocity exists for airborne asbestos to be captured by air filtration devices.
The current practice in the asbestos industry is to specify four air changes per hour for the work enclosure. However, determination of ventilation requirements based upon air changes is generally viewed as an unacceptable criteria by ventilation engineers, but is an unfortunate convenience due to the variability of exposure in asbestos work and the nature of changing work sites as previously discussed. The four air changes per hour "standard" is best viewed not as a method of controlling exposure to workers, but rather as a guideline to exhaust a sufficient quantity of air to maintain the negative static pressure within the work area. The guideline static pressure differential of 0.02 inch w.g. has also become standard, and is generally accepted as sufficient since this will produce noticeable drafts around windows, doors, etc. in general building ventilation. An effectively contained asbestos removal zone should contain only small leaks (if any), and with a draft initiated by a 0.02 inch w.g. differential, escape of airborne asbestos through such openings should be prevented. The fact that the exhaust from the air filtration units is filtered enables discharge of uncontaminated air to the surroundings, but does not necessarily relate to any appreciable reduction in work exposure within the contained work area during active removal.